Field of the Invention
The present invention relates to a thermal image recording apparatus which comprises a thermal print head.
In the thermal printing process, a dye-bearing donor ribbon is brought into contact with a dye-receiving print sheet at a print zone. Thermal printing is effected by contacting the donor ribbon which is advanced usually in step-wise fashion in a given direction, e.g. from a supply spool to a take-up spool, with a multi-element print head which spans the ribbon in a direction transverse to the direction of ribbon travel. The print head typically comprises a linear array of closely spaced resistive heating elements, each being. independently addressable by an applied voltage to heat that portion of the donor ribbon directly opposite and thereby cause dye to transfer from the ribbon to the print sheet. To maintain intimate contact between ribbon and print sheet during this printing operation, the donor ribbon and print sheet are partially wrapped over the surface of a rotatably driven print drum. The print drum is usually driven by a precision stepper motor to maintain precise synchronisation with the printing of information on the print sheet, whereas the take-up spool is rotatably driven by a far less expensive DC motor, its function being simply to collect expended donor ribbon. The donor ribbon is supplied by a rotatably mounted supply spool, and a clutching arrangement is used to control the drag on the ribbon by the supply spool so as to prevent free-wheeling of the supply spool under the influence of the take-up spool motor. In colour thermal printers, the donor web usually comprises patches of cyan, yellow and magenta dyes in a repeating series, and the print-receiving sheet is passed three times through the print zone to receive a full-colour image,
The printing process described hereinbefore can be used for producing opaque as well as transparent prints. The former are prints on white or coloured paper that are intended for direct reading, whereas the latter are mostly so-called overhead projection prints intended for optical projection on a screen.
Each of the two types of print-receiving sheets requires a particular setting of the thermal image-recording apparatus.. For instance, a transparent print-receiving sheet requires the use of a dye-bearing ribbon with a higher dye content than the one which is required for printing on opaque sheets. This is due to the fact that light passes twice through the coloured image layer on an opaque sheet whereas in a transparent sheet it passes only once.
Further, there are a number of adjustments on the image recording apparatus that also depend on the type of print-receiving sheets used. One such adjustment relates to a drive control capable of controlling application energy given to heater elements in the thermal head on the basis of not only history information of a given heater element but also print information including print history information of heater elements adjacent to the present heater element. A print information-processing is carried out based on the print history information of the given heater element, the print history information of the adjacent heater elements, and the print history information of the preceeding and two times before print information of the adjacent heater elements.
In another adjustment, image data is extracted at a certain interval, and the heating resistor elements of the print head are supplied with electric energy corresponding to a density specified by the image data so as to correspond to the data extraction interval. The image data is extracted at an interval amounting to at least one single pixel out of the image data fed from an external device in the main scanning direction so as to prevent thermal interference with the neighbouring heating resistor elements, so that the heating resistor elements are operated accurately with respect to such electric energy so as to form a dot larger than the dot specified by the highest density of the inputted image data. As a result, density can be expressed in a high number of gradations.
All these and still other adjustments determine a given setting of the apparatus for a given type of print-receiving sheet and a given type of dye-bearing ribbon, and it will be understood that if an operator changes the cassette only of the apparatus that contains the print-receiving sheets for the production of another type of prints, while the other settings of the apparatus remain unaltered, the operation of the apparatus will not be satisfactory.
Also, an automatic adjustment of the apparatus by the reading of a code provided on the cassette with print-receiving sheets as practiced e.g. in the field of amateur photography for reading the film sensitivity on the casing of a film cassette and setting the camera accordingly cannot be used in this type of apparatus, since replacement of an opaque print-receiving material by a transparent one requires also replacement of the cassette for the dye-bearing ribbon.